The present invention relates to a base station and packet transfer equipment in a mobile network utilizing T1 leased lines using multilink PPP as the backhaul lines of a mobile telephone base station.
A mobile network usually uses T1 leased lines multilinked to backhaul lines from the base station accessed by mobile terminals to an IP network. Since T1 leased lines are laid in a broad range, they are suitable for base stations of mobile communication. Channels are multiplexed by multilinking T1 leased lines to enhance the data transmission speed. Base stations connected to a router by T1 lines to which multilink PPP is applied and the router collectively managing the base stations will be described with reference to FIG. 1. FIG. 1 is a block diagram of a mobile network. In FIG. 1, a mobile network 100 includes a mobile terminal 10, two base stations 20 and a router 30 collectively managing the base stations 20. Each of the base stations 20 has a data transceiving unit 21 which exchanges data with the mobile terminal 10 and a backhaul line interface 22 which exchanges data with the router 30. The router 30 has a line interface 31 connected to backhaul line interfaces 22 of the base station 20 and to an IP data network 40.
T1 lines 50 to which multilink PPP connecting the base stations 20 with the router 30 is applied secure broadband communication paths by collectively managing data communication paths of lines set in the same multilink group into a logically single line.
The method of packet transmission using multilink PPP (MP) between a base station and the router will be described below with reference to FIG. 2. FIG. 2 here is a diagram illustrating the method of packet transmission using multilink PPP between the base station and the router. Referring to FIG. 2, the base station 20, when it receives a packet at its IP unit 23 from a terminal, fragments the IP packet in its MP unit 24 according to the band-pass situation of each link, and allocates the fragments to lines 51 constituting the multilink for transmission. At the router 30 on the receiving side, an MP unit 34 reassembles the fragments to reconstruct the IP packet, which is transmitted from an IP unit 33 to an IP data network.
Multilink encapsulation and overhead will be described now with reference to FIG. 3. Here, FIG. 3 is a diagram illustrating multilink encapsulation and overhead. Referring to FIG. 3, the base station splits an IP packet 60 into fragment 1 through fragment 4. Each of the resultant fragments, to which one-byte flags 61 at the leading and trailing edges, a usual PPP header 62, an MP header 63 having information for order control and a frame check sequence (FCS) 64 are added, is sent to each PPP link. The overhead size increment due to multilink encapsulation is 12 bytes×the number of links.
The mobile network is divided into data communication lines and voice communication lines. The data line network, architected mainly for use by e-mail, is unsuitable for applications which require real time links, such as voice communication.
In a fixed communication network, on the other hand, VoIP is realized also for voice communication by dedicated hardware such as IP telephone sets. Although VoIP using dedicated hardware uses the same lines as data communication by personal computers and the like, speech quality is warranted by subjecting VoIP packets to prioritized control. In a fixed communication network, VoIP can be more easily realized because a broader band is provided than for a mobile network.
Methods of prioritized control of VoIP packets include packet prioritizing control by setting a type of service (ToS) value of the IP header. The ToS area of the IP header will be described with reference to FIG. 4. FIG. 4 here shows the format of the IP header. Referring to FIG. 4, an IP header 400 includes a version 401, a header length 402, a ToS 403, a datagram length 404, an identification number 405, a flag 406, a fragment offset 407, a time to live (TTL) 408, a protocol number 409, a check sum 410, a source IP address 411 and a destination IP address 412. The ToS 403 consists of eight bits, of which the leading three bits denote the level of priority.
The dedicated VoIP hardware sets the ToS value high and transmits it to a host unit. The host unit references the ToS value of the packet and performs prioritized control accordingly. The host unit stores received packets into transmit queues according to the ToS value. Packet transmission from the queues is accomplished in a prioritized manner in the descending order of the ToS value. If any data remains in a queue of high priority, no data will be transmitted from queues lower in priority.
JP-A-2004-056336 discloses a VoIP system which realizes seamless handover with little delay between mobile terminals and a base station.
Conventionally, a mobile network uses separate channels for data communication and speech communication. In recent years, the possibility of introducing a VoIP service in a mobile network which realizes speech communication by data communication has been contemplated.
However, the data delay time permissible in a mobile network is far shorter than in data communication, and accordingly it is difficult to warrant the quality of VoIP between existing data communication devices. For this reason, it is necessary to enhance the quality of VoIP packets between mobile network devices.
As a way to warrant the quality of VoIP packets, the packets can be placed under prioritized control. In accomplishing prioritized control of packets, the ToS value of the IP header is set on the transmitting side, and the packet transmitting device of the receiving host unit references ToS values and transmits packets in the descending order of the set ToS value. In the mobile network described with reference to FIG. 1 as well, if prioritized control of packets is used, the quality of VoIP can be warranted by assigning levels of priority at the terminal to packets to be transmitted to the base station and transmitting on the base station side packets to a router which gives top priority to VoIP packets. However, since the packet data channel of the mobile network is not sufficient in bandwidth and moreover uses the same line as other sets of data, if the line is congested with non-VoIP packets or the output interface is congested, the VoIP packets may be delayed, resulting in a failure to obtain sufficient quality assurance.
A case in which VoIP packets are delayed in the mobile network shown in FIG. 1 even when packets are subjected to prioritized control will be described with reference to FIGS. 5A to 5C. FIGS. 5A to 5C here are diagrams illustrating how VoIP packets are delayed even when packets are subjected to prioritized control. Referring to FIG. 5A, for packets 501 and 506 transmitted from terminals, packet priority levels are set at mobile terminals in the ToS areas of their respective IP headers. Here, the priority level of VoIP packets is set to 4. Numerals “4”, “3”, “2” and “1” in FIG. 5 (and FIG. 10 to be referenced afterwards) represent priority levels, the relationship among which is 4>3>2>1.
Having received packets, the base station queues the packets in accordance with the set levels of priority. A queue 510-4 is the queue of the highest priority level, a queue 510-3 is the queue of the second highest priority level, a queue 510-2 is the queue of the third highest priority level, and a queue 510-1 is the queue of the lowest priority level. The packets are transmitted to the router in the descending order of the priority level, and a packet higher in priority level is transmitted to the router with a less delay. However, supposing a case in which the packet 506 of a large size and “3” in priority level is queued in a state in which no packet is in the queue 510-4 for VoIP as shown in FIG. 5B, the VoIP packet 501 will not be transmitted until the transmission of the large size packet 506 is fully completed even if the VoIP packet 501 is queued afterwards as shown in FIG. 5C. As a result, transmission of the VoIP packet is delayed even if prioritized control of packets is performed.